1. Field of the Invention
The present invention relates to a system for controlling the temperature of a cylinder wall in an engine to heat and cool the cylinder wall of the engine, or to provide an appropriate temperature profile to the cylinder wall of the engine in order to reduce the frictional resistance of a piston.
2. Description of the Related Art
A cooling circuit or a water jacket has been proposed in recent years for use in an engine, which is designed so that its ability to cool a portion around a combustion chamber and an upper portion of a cylinder liner is increased to inhibit the knocking. Additionally, over-cooling of the lower portion of the cylinder liner is prevented to reduce the frictional resistance of the piston. Both effects allow for an increase in engine output and an improvement in the specific consumption of fuel.
For example, Japanese Patent Application Laid-open No.1-227850 describes an engine in which a groove-shaped circulation chamber is provided for the circulation of cooling water, which is defined in an upper portion of a cylinder liner to enhance the cooling ability. This prevents the seizure of the piston, prevents the leakage of gas and inhibits knocking. The Japanese Patent Application also discloses a convection chamber for natural convection of the cooling water, where the chamber is provided in a lower portion of the cylinder liner to prevent the over-cooling, thereby providing a reduction in the frictional resistance of the piston.
Japanese Patent Application Laid-open No.3-67052 describes an engine, which is designed so that an upper portion of a cylinder liner is cooled by means of a water jacket, and includes a space defined in a lower portion of the cylinder liner to communicate with a crank chamber, thereby preventing the over-cooling.
Frictional losses at slide portions of the cylinder liner and the piston are produced by (1) a frictional resistance due to the shearing of an oil film of a lubricating oil generated by the sliding movement of a piston ring and (2) the drag resistance of a surplus amount of the lubricating oil adhered to the cylinder liner. Therefore, if the viscosity of the lubricating oil is reduced to as low a value as possible, in a range enough to maintain an oil film forming ability, the friction loss is decreased. Hence, it is desirable that the temperature of the slide portions be increased to reduce the viscosity of the lubricating oil. For this purpose, it is a conventional practice to prevent the over-cooling of the lower portion of the cylinder liner (the position between an intermediate portion and a bottom dead center of the piston) by altering the structure of the cooling circuit or the structure of the water jacket. The internal pressure in the lower portion of the cylinder liner is low and hence, the lubricating condition is not severe, and the lower portion of the cylinder liner can be adjusted to a temperature higher than that in the prior art to decrease the frictional losses. In the prior art, however, the ability to cool the lower portion of the cylinder liner is merely reduced, and the lower portion is not positively heated. Therefore, the frictional losses are not sufficiently reduced.
Accordingly, it is an object of the present invention to provide an appropriate temperature profile to the cylinder wall to minimize the friction loss produced at the slide portions of the cylinder wall and the piston.
To achieve the above object, according to a first aspect of the present invention, there is provided a system for controlling the temperature of a cylinder wall in an engine, comprising a heating means for heating at least a portion of a cylinder wall in a vicinity of a bottom dead center of a piston which is slidably guided in the cylinder wall; a cooling means for cooling at least the portion of the cylinder wall in the vicinity of the bottom dead center of the piston, a temperature detecting means for detecting a temperature of the cylinder wall, and a control means for controlling the heating means and the cooling means, based on the detected temperature of the cylinder wall, so that the temperature of the cylinder wall is brought into a target temperature.
With the above arrangement, the heating means and the cooling means for heating and cooling at least the portion of the cylinder wall in the vicinity of the bottom dead center of the piston, and are controlled by the control means, so that the temperature of the cylinder wall is brought into the target temperature. Therefore, the temperature of the portion of the cylinder wall in the vicinity of the bottom dead center of the piston can be brought into a sufficiently high temperature in such a range enabling an oil film of a lubricating oil to be ensured, and the viscosity of the lubricating oil can be decreased to minimize the friction loss between slide portions of the cylinder wall and the piston, thereby providing an increase in engine output, a reduction in amount of fuel consumed and a reduction in amount of lubricating oil consumed.
According to a second aspect of the present invention, the heating means heats the cylinder wall by an exhaust gas flowing through a gas jacket defined in a cylinder block, and the cooling means cools the cylinder wall by fresh air flowing through the gas jacket defined in the cylinder block.
With the above arrangement, the cylinder wall is heated by permitting the exhaust gas to flow through the gas jacket, and cooled by permitting the fresh air to flow through the gas jacket. Therefore, the temperature of the cylinder wall can be increased quickly and controlled properly to a desired temperature.
According to a third aspect of the present invention, the heating means comprises a heat transfer member for transferring the heat of an exhaust gas to a cylinder block, and the cooling means cools the heat transfer member by cooling water flowing through a water jacket defined in the heat transfer member.
With the above arrangement, the heat of the exhaust gas is transferred to the cylinder block through the heat transfer member to heat the cylinder wall, and cooling water is permitted to flow through the water jacket defined in the heat transfer member to cool the heat transfer member, thereby cooling the cylinder wall. Therefore, the temperature of the cylinder wall can be increased quickly and controlled properly to a desired temperature.
According to a fourth aspect of the present invention, there is provided a system for controlling the temperature of a cylinder wall in an engine, comprising an upper water jacket facing a portion of a cylinder wall on which a piston is slidably guided, where the upper water jacket is adjacent to a top dead center of the piston, a lower water jacket facing another portion of the cylinder wall, which is adjacent to a bottom dead center of the piston, an upper cylinder portion temperature detecting means for detecting a temperature of an upper portion of the cylinder wall adjacent to the top dead center of the piston, a lower cylinder portion temperature detecting means for detecting a temperature of a lower portion of the cylinder wall adjacent to the bottom dead center of the piston, an upper cooling circuit for controlling cooling water flowing through the upper water jacket to converge the temperature of the upper portion of the cylinder wall detected by the upper cylinder wall temperature detecting means to a target temperature for the upper portion of the cylinder wall, and a lower cooling circuit for controlling the cooling water flowing through the lower water jacket to converge the temperature of the lower portion of the cylinder wall detected by the lower cylinder wall temperature detecting means to a target temperature for the lower portion of the cylinder wall.
With the above arrangement, the cooling water flowing through the upper water jacket facing the portion of the cylinder wall adjacent to the top dead center of the piston is controlled by the upper cooling circuit, and the cooling water flowing through the lower water jacket facing the portion of the cylinder wall adjacent to the bottom dead center of the piston is controlled by the lower cooling circuit. Therefore, the temperature of the upper portion of the cylinder wall liable to be subjected to a heat load produced by the combustion and the temperature of the lower portion of the cylinder wall that may or may not be subjected to the heat load produced by the combustion can be controlled individually. Thus, at the portion of the cylinder wall adjacent to the top dead center of the piston, the over-heating of the engine can be prevented to maintain the temperature of an oil film at an appropriate point, while preventing an abnormal combustion, thereby decreasing the frictional force to decrease the friction loss. In addition, at the portion of the cylinder wall adjacent to the bottom dead center of the piston, the temperature of the oil film can be increased up to a point as high as possible in such a range that the breaking of the oil film does not occur, thereby reducing the viscosity to provide an increase in engine output, a reduction in amount of fuel consumed and a reduction in amount of lubricating oil consumed.
According to a fifth aspect of the present invention, the lower cooling circuit includes a heat exchanger for heating the cooling water by the heat of an exhaust gas.
With the above arrangement, the heat exchanger for heating the cooling water by the heat of the exhaust gas is provided in the lower cooling circuit. Therefore, it is possible to heat the cooling water by utilizing the heat of the exhaust gas without provision of a special heat source to contribute to a reduction in cost.
According to a sixth aspect of the present invention, the lower cooling circuit includes a heat exchanger for heating the cooling water by the heat of an electric heater.
With the above arrangement, the heat exchanger for heating the cooling water by the heat of the electric heater is provided in the lower cooling circuit. Therefore, it is possible to heat the cooling water before starting the engine to increase the temperature of the lower cylinder wall, thereby contributing to a decrease in friction loss and an improvement in emission.
According to a seventh aspect of the present invention, there is provided a system for controlling the temperature of a cylinder wall in an engine, comprising an upper water jacket facing a portion of a cylinder wall on which a piston is slidably guided, where the upper water jacket is adjacent to a top dead center of the piston, and a lower water jacket facing another portion of the cylinder wall adjacent to a bottom dead center of the piston, wherein cooling water exiting a radiator is passed through the upper water jacket and then through the lower water jacket back to the radiator.
With the above arrangement, the cooling water passed through the upper water jacket facing the portion of the cylinder wall adjacent to the top dead center of the piston and having an increased temperature is supplied to the lower water jacket facing the portion of the cylinder wall adjacent to the bottom dead center of the piston. Therefore, the temperature of the lower cylinder wall has been increased up to a point higher than that in the prior art in which the cooling water is permitted to flow from a lower portion of a cylinder block toward an upper portion of the cylinder block. Thus, the temperature of an oil film at the portion of the cylinder wall adjacent to the bottom dead center of the piston can be brought into a point as high as possible to reduce the viscosity, and the frictional force can be decreased to provide an increase in engine output, a reduction in amount of fuel consumed and a reduction in amount of lubricating oil consumed.
According to an eighth aspect of the present invention, the cooling water passed through the upper water jacket is supplied to a site corresponding to each cylinder at a lower end of the lower water jacket through a gallery.
With the above arrangement, the cooling water supplied from the upper water jacket to the lower water jacket flows independently into each of the cylinders through the gallery. Therefore, the temperatures of the walls of the cylinders can be equalized to decrease the fluctuation in combustion and the variation in torque. Moreover, the gallery communicates with the lower end of the lower water jacket and hence, when the cooling water is poured into the lower water jacket, the withdrawal of air is improved.